This invention relates to an electro-optical device having a large number of pixels for use in a personal computer display, a hand-held computer display, displays of various measuring instruments, television receivers, printer shutters, and so forth.
The inventors of the present invention have heretofore developed a non-linear resistance element or a--Si TFT using a silicon nitride film, a silicon oxide film, a silicon nitride oxide film or a silicon carbide film as a non-linear resistance thin film. These are described, for example, in Japanese Patent Laid-Open No. 90192/1986 and No. 94086/1986
Since these electro-optical switching elements using a--Si as the base material are formed by chemical reaction between different gases in a plasma CVD apparatus to deposit a film, H mixes unavoidably into the base a--Si film. To produce a non-linear resistance element having SiN.sub.x (N/Si=0.4.about.0.8), for example, an SiH.sub.4 gas is reacted chemically with an N.sub.2 gas or an NH.sub.3 gas to form the film so that the decomposed H component of the SiH.sub.4 gas or the H component of NH.sub.3 mixes unavoidably in an amount of 10 to 20% into the resulting SiN.sub.x film. When a non-linear resistance element of SiO.sub.x is formed, too, the SiH.sub.4 gas is used likewise so that 10 to 20% of the decomposed H component of the SiH.sub.4 gas mixes into the SiO.sub.x film.
Since the switching element using H-containing amorphous silicon (hereinafter referred to as "a--Si:H") as the base material exhibits a photoelectric effect peculiar to a--Si:H, the electric characteristics of the element change depending on the atmosphere (brightness and darkness) around the element. In the case of a non-linear resistance element (see FIGS. 1 and 2) having a structure consisting of transparent pixel electrodes, a wiring electrode and a non-linear resistance film using a--Si:H as the base material and interposed between the former two, the voltage-v-current characteristics between the wiring electrode and the transparent pixel electrode change depending on the brightness and darkness around the element as shown in FIG. 3. If this non-linear resistance element is used for a liquid crystal display such as shown in FIG. 6, a difference of contrast occurs between the bright atmosphere and the dark atmosphere. This reason will be described next.
FIG. 7 is a circuit diagram of a liquid display device using the non-linear resistance element. Reference numeral 13 represents a wiring electrode made of a metal, 17 is a transparent electrode, 16 is a liquid crystal and 15 is a non-linear resistance element. FIG. 8 is an equivalent circuit diagram of one pixel, and symbols C.sub.1 and R.sub.1 represent the capacitor capacity and the resistance of each non-linear resistance element. C.sub.LC and R.sub.LC represent the capacitor capacity and the resistance of each liquid crystal. The driving waveform in the matrix liquid crystal panel shown in FIG. 7 is a time division driving waveform shown in FIGS. 9(A) and 10(A) by way of example. When a voltage V.sub.op is applied between A and C of FIG. 8, that is, between the non-linear resistance element and the liquid crystal pixel, for a period T.sub.o, the charge is stored in the capacitor C.sub.LC of the liquid crystal and thereafter, the voltage is applied between A and B and between B and C due to resistance division by R.sub.1 and R.sub.LC for the period T-T.sub.o. If the resistance of R.sub.1 is low in the period T-T.sub.o, the charge stored in C.sub.LC in the period T.sub.o leaks from B to A, that is, through the non-linear resistance element. In short, the area of the oblique line portion at the lower part of FIGS. 9(B) and 10(B) changes with R.sub.1. If the value R.sub.1 is small, the area of the olique line portion is small and the voltage applied between B and C, that is, the voltage applied to the liquid crystal, is small.
Therefore, in the case of the liquid crystal display whose non-linear resistance element has the electric characteristics such as shown in FIG. 3, the resistance value R.sub.1 changes in the dark/bright atmosphere in the low voltage region in the period T-T.sub.o so that the voltage value applied to the liquid crystal changes and the difference of contrast occurs consequently. In the worst case, the characters that have been displayed in the dark atmosphere would disappear and would not be able to be seen if they are abruptly moved to the bright atmosphere. FIG. 10(A) and (B) show the example of the waveform applied between A and C and the waveform of the B--C voltage at the time of OFF.
In an electro-optical device having a switching element using a--Si as its base material, the present invention inhibits the base a--Si film from containing substantially H to eliminate the photoelectric effect (FIG. 4) and to prevent the drop of contrast of the electro-optical device at a bright place.